1. Field of the Invention
The present invention relates to an objective lens used for an endoscope.
2. Description of the Related Art
In the field of medical care, an endoscope is conventionally used to observe a site of a patient that is difficult to externally observe in treating and/or diagnosing the inside of the body of the patient. In recent years, with the reduced diameters of endoscopes typified by nasal endoscopes, small-sized image pickup elements (for example, CCDs and CMOSs) for endoscopes have been developed, with the pixel pitches thereof reduced year by year. Accordingly, objective lenses for endoscopes need to have reduced sizes and to meet optical performance requirements for an increase in the angle of view, aberration correction, prevention of a decrease in the quantity of light, and the like.
Conventionally proposed objective lenses for endoscopes are described in, for example, Japanese Laid-Open Patent Applications 2004-61763, 2004-354888, Hei 8-122632, and Hei 10-20189.
The objective lens for endoscopes described in Japanese Laid-Open Patent Application 2004-61763 is intended to offer a wide angle of view of about 135 to 140 degrees and a required back focus, while properly correcting aberration, particularly the chromatic aberration of magnification to exhibit acceptable optical performance. To accomplish this object, the objective lens is characterized as follows. The objective lens has a four-group, five-lens configuration, that is, the objective lens is composed of a cemented lens L4+5 made up of a negative first lens L1 with a concave surface directed toward an image side, a positive second lens L2 with a plane directed toward the image side, a brightness aperture stop, a positive third lens L3 with a plane or a surface of a large radius of curvature directed toward an object side, and a cemented lens including a positive fourth lens L4 with a plane or a surface of a large radius of curvature directed toward the object side and a fifth lens L5 composed of a negative meniscus lens. The first lens L1, the second lens L2, the brightness aperture stop, the third lens L3, and the cemented lens L4+5 are arranged in this order so that the first lens L1 is closest to the object. The cemented lens L4+5 as a whole has a positive refractive index. The objective lens satisfies conditions (21) to (23):1.50|d/f1′|<2.50  (21)0.96<f′/h<1.04  (22)12.0<f′×(v4−v5)/(Rc×Bf′)  (23)where d denotes the distance from the top of the concave surface of the first lens L1 to the brightness aperture stop, f1′ denotes the focal length of the first lens L1, h denotes a maximum image height, f′ denotes the focal length of the whole system, Bf′ denotes a back focus, v4 denotes the Abbe number of the positive fourth lens L4, v5 denotes the Abbe number of the negative fifth lens L5, and Rc denotes the absolute value of the radius of curvature of the cemented surface of the cemented lens L4+5.
Japanese Laid-Open Patent Application 2004-354888 is intended to provide an objective lens for endoscopes which has a wide angle of view, a small outer diameter, and a first lens with a small maximum ray height and which is further suitable for small-sized CCDs. To accomplish this object, the objective lens is characterized as follows. The objective lens is composed of a front lens group and a rear lens group, between which an aperture stop is arranged. The front lens group is composed of a first lens with a negative refractive power and a second lens with a positive refractive power having a surface of a small radius of curvature directed toward an object; the first lens and the second lens are arranged in this order so that the first lens is closer to the object. The rear lens group is composed of a third lens with a positive refractive power having a surface of a small radius of curvature directed toward the object side, a fourth lens with a positive refractive power, and a fifth lens with a negative refractive power; the fourth lens and the fifth lens are cemented together. When f denotes the composite focal length of the whole system, and f3 denotes the focal length of the third lens, the objective lens satisfies condition (24).2.0<|f3/f|<3.0  (24)
The objective lens for endoscopes described in Japanese Patent Laid-Open Hei 8-122632 is intended to enable a reduction in costs required to polish and assemble the lens with proper lens performance maintained. To accomplish this object, the objective lens is characterized as follows. The objective lens includes a negative first lens L1 with a concave surface directed toward an image side, a positive second lens L2 with a plane directed toward the image side, a brightness aperture stop, a positive third lens L3 with a plane directed toward an object side, and a fifth lens L5 made up of a positive fourth lens L4 with a plane directed toward the object side and a negative meniscus lens. The first lens L1, the second lens L2, the brightness aperture stop, the third lens L3, and the fifth lens L5 are arranged in this order so that the first lens L1 is closest to the object, and the fourth lens L4 and the fifth lens L5 make up a cemented lens with a positive refractive power as a whole. Moreover, the objective lens satisfies conditions (25), (26), and (27):0.75<|f1×Bf/f2|<1.45  (25)Di<Ri  (26)v2<45  (27)where f1 denotes the focal length of the first lens L1, Bf denotes the back focus of the whole system (L1 to L5), f denotes the composite focal length of the whole system (L1 to L5), Di denotes the central thickness of the lens Li with a concave surface, Ri denotes the radius of curvature of the convex surface of the lens Li with the convex surface, and v2 denotes the Abbe number of the second lens L2.
The objective lens for endoscopes described in Japanese Laid-Open Patent Application Hei 10-20189 is intended to inhibit a possible excessive decrease in the radius of curvature of the front group lens and to offer a wide angle of view while correctively reducing distortion with a sharp decrease in the quality of ambient light prevented. To accomplish this object, the objective lens is characterized as follows. The objective lens is composed of a first lens group that as a whole has a negative refractive power, a brightness aperture stop, and a second lens group that as a whole has a positive refractive power, the first lens group, the brightness aperture stop and the second lens group being arranged in this order so that the first lens group is closest to the object. The first lens group is composed of an object-side first lens and an image-side second lens which both have a negative refractive power. The objective lens satisfies conditions (28) and (29):−5.0<q1<−0.9  (28)−0.5<q2<9.0  (29)where q1 denotes the shaping factor of the first lens (=(r2+r1)/(r2−r1), q2 denotes the shaping factor of the second lens (=(r4+r3)/(r4−r3), r1 denotes the radius of curvature of an object-side surface of the first lens, r2 denotes the radius of curvature of an image-side surface of the first lens, r3 denotes the radius of curvature of an object-side of the second lens, and r4 denotes the radius of curvature of an image-side of the second lens.
As described above, the objective lenses for endoscopes are described in, for example, Japanese Laid-Open Patent Applications. 2004-61763, 2004-354888, Hei 8-122632, and Hei 10-20189 all have what is called a retro focus type lens configuration in which the lens closest to the object or the front lens group closer to the object than the aperture stop has a negative refractive power, whereas the rear lens group located closer to the image than the aperture stop has a positive refractive power. The objective lenses further satisfy the predetermined combinations of the conditions shown in conditions (21) to (29).
To offer a somewhat wide angle of view, the retro focus type lens configuration needs to be such that the lens closest to the object includes a concave surface such that the lens has negative refractive power. The retro focus type objective lens, adopted as an objective lens for endoscopes, needs to have a further reduced lens outer diameter in order to meet a demand for a further reduction in the diameter of the endoscope than that of the conventional endoscope, while offering a wider angle of view, which is required for the endoscope. To achieve this, the power of the lens in the front lens group, that is, the negative power of the lens closest to the object, may be increased (i.e., strengthened). However, when the negative power of this lens is increased in association with the further reduced diameter of the endoscope, the curvature of the concave surface of this lens increases. Then, this lens offers degraded processibility and has a high negative refractive power. Thus, possible defocusing of the lens in a frame significantly affects an image. This results in a phenomenon called partial defocusing in which a peripheral portion of the image provided by the endoscope is blurred.